Reduce legacy check to one line

Because effected users would have this library loaded in other plugins,
there isn't any point in running `deactivate_plugins` or `wp_die`.

By requiring WordPress 3.2, we also inherantly require PHP5,
so check of PHP_VERSION is not needed.

By checking `$wp_version` outside `gpu_git_plugin_updates_init`, we
can combine all checks into one `if` statement, and don't need
to declare `global $wp_version`.

Because users not running WordPress older than 3.2 *already* have
an update nag displaying, there's no point in displaying an
update notice. Because they're already habitually ignoring upgrades,
it's okay for this plugin to silently not do anything until
WordPress core is upgraded.

Ideas for this commit came about while working through
@afragen's [pull request](https://github.com/afragen/git-plugin-updates/commit/d0315f6c95711d2ccece5068b94536f152b17242) for issue #12.